The present invention relates to stator blading of return channels for two-dimensional centrifugal stages of a multi-stage centrifugal compressor with improved efficiency.
A centrifugal compressor is a machine into which there is admitted a compressible fluid, which is then discharged at a pressure greater than its own intake pressure.
Centrifugal compressors are formed by a cylindrical body or case which is closed at its own ends and contains the operative units of the machine.
Centrifugal compressors can include one or more stages, and can be used for medium and/or high pressures, but hereinafter in the present description reference is made to multi-stage centrifugal compressors.
Non-exhaustive examples of the possible uses of such centrifugal compressors are: re-injection of gas, re-compression of gas, use of the compressor in association with plants for supply of gaseous fuel in machines for generation of power, in refineries, in plants for synthesis of methanol and ammonia, and in high-pressure or liquid natural gas lines.
The basic elements which constitute a multi-stage centrifugal compressor are a rotor which rotates around the axis of the machine, and a series of diffusers or stators with return channels between the various stages, which are integral with a case.
The rotor consists of a series of discs with blading fitted onto a single shaft.
In the rotor, the fluid, which is collected from the central section of the rotor, is forced by this rotary blading and undergoes an increase in speed and therefore in dynamic pressure. The static pressure is also increased owing to the divergent profile of the spaces between the blades of the blading. The rotor therefore gives rise to an increase in the total pressure and an increase in the speed.
Each disc of the rotor is followed by a diffuser, which is also provided with blading.
In the diffuser, owing to the divergent shape of the ducts between one blade and another, the fluid undergoes a reduction of speed. This leads to the loss of part of the dynamic pressure gained in the preceding disc of the rotor, and to an increase in the static pressure, caused by the transformation of kinetic energy into pressure energy.
With each diffuser there is associated a return channel, which conveys the fluid to the successive disc of the rotor. More specifically, the flow of the fluid is returned in the axial direction, thus eliminating the tangential speed imparted by the rotor disc of the preceding stage.
The assembly of a disc of the rotor and of a diffuser, with the corresponding return channel, constitutes a stage, which is separated from the adjacent stages by annular diaphragms.
In compressors of this type there is however the difficulty of conveying the fluid satisfactorily from the output of one stage to the intake of the subsequent one, such as to limit the losses.
In general the return channels are characterised by geometries of a simple type. Their profile is frequently constituted by arcs of a circle which are regularly connected.
The aerodynamic design of these components is carried out using criteria which are substantially empirical, and in general design methods which are aimed specifically at reduction of the losses are not used.
However the return channels give rise to very complex aerodynamic phenomena, which could potentially introduce significant losses of efficiency.
A detailed analysis of the flow in the return channels can be carried out both with experimental means and numerical methods.
This shows that these channels are frequently affected by effects of separation of the flow, which detract from the overall efficiency of the compressor.
The object of the present invention is to eliminate the above-described disadvantages and in particular to provide stator blading of return channels for two-dimensional centrifugal stages of a multi-stage centrifugal compressor with improved efficiency, which makes it possible to increase the overall performance of the compressor.
Another object of the present invention is to provide stator blading of return channels for two-dimensional centrifugal stages of a multi-stage centrifugal compressor with improved efficiency, which is particularly reliable, simple, functional and has relatively low costs.
These objects and others according to the present invention are achieved by providing stator blading of return channels for two-dimensional centrifugal stages of a multi-stage centrifugal compressor with improved efficiency, as described in claim 1.
Further characteristics are indicated in the subsequent claims.
Stator blading of return channels for two-dimensional centrifugal stages of a multi-stage centrifugal compressor with improved efficiency has the advantage of comprising a significant reduction in the areas of separation in the vicinity of the output cross-section of the blades of this blading. This therefore provides improved polytropic stage efficiency.
The characteristics and advantages of stator blading of return channels for two-dimensional centrifugal stages of a multi-stage centrifugal compressor with improved efficiency according to the present invention will become more apparent from the following description provided by way of non-limiting example with reference to the attached schematic drawings, in which: